Estimating Monetized Benefits of Groundwater Recharge from Stormwater Retention Practices

Stormwater impacts from development have been documented extensively in peer-reviewed literature and summarized in the National Research Council’s report titled Urban Stormwater Management in the United States (National Research Council, 2009). To address these impacts, the U.S. Environmental Protection Agency (EPA) Office of Water (OW) evaluated several potential scenarios for managing stormwater from new development and redevelopment using small stormwater retention practices. The objective of these practices is to reduce runoff volume to simulate pre-development hydrology and protect water quality. These practices are often designed to infiltrate the retained water, which can provide groundwater recharge to help maintain predevelopment groundwater levels. For the purposes of this study, infiltration was assumed, whereas in practice capture-and-use or other methods also may be applied.

The purpose of the study is to inform valuation of groundwater recharge from stormwater retention in areas projected for new development and redevelopment. Retention practices can prevent the loss of natural recharge that may occur on a developed site without these practices. This study examined a simplified methodology for estimating recharge volume and used observed prices of water for monetary valuation.

Groundwater recharge is just one benefit within the larger context of the costs and benefits of stormwater retention practices. The full range of potential benefits were not addressed in this study, but the full suite of benefits should be considered when comparing to an estimated cost of implementation. Costs of implementing retention practices in areas of new and redevelopment also were not addressed in this study.

The estimates represent the groundwater recharge volume and value that would result if stormwater retention practices were implemented in areas within the contiguous U.S. not currently requiring stormwater retention. The stormwater retention practices were assumed to capture a specified depth of runoff, in the range of 0.5 to 2 inches (i.e. not large storm attenuation) with a goal of reducing runoff volumes to levels similar to those for pre-developed landscape conditions. This approach is commonly referred to as “green infrastructure” for stormwater management or low impact development (LID). While small storm retention that approximates predevelopment runoff could also be achieved with cisterns in those areas where infiltration is not desired or is not feasible, the option of cistern capture was not evaluated in this study. The timeframe selected for the purpose of the predictive study was the 20-year period of 2021 through 2040.